The present invention relates to an interconnection board, in particular, a multi-layered printed circuit board, so treated as to prevent a warp occurring due to heat evolved in a refolding furnace and a method for manufacturing the same, and further to an electric apparatus, in particular, a portable compact electric apparatus, manufactured using such a interconnection board.
A conduction pattern of an interconnection board with electronic components mounted thereon is pattern-designed based on a product configuration and electric circuit design. According to the conduction pattern, a copper-clad laminate plate is subjected to, for example, a subtractive process whereby those unrequired portions of a copper foil on the laminate place are dissolved/removed with chemicals to obtain a necessary conduction pattern on the laminate plate.
In order to mount electric components on the resultant interconnection board and, by doing so, manufacture a circuit board, soldering is performed on the components. In the case where extensively used chip components are to be mounted on the electronic apparatus, use has usually been made of a reflow soldering.
The reflow soldering has been done by conveying a circuit board 14 with electronic components 13 mounted thereon on a conveying belt (12a, 12b) and into a reflow furnace 10 as shown in FIG. 7. First, a cream solder is coated onto pads on an interconnection board 1 and then terminal sections of the electronic components 15, such as chip components, are accurately positioned and mounted on the pads on an electronic conduction layer of the printed circuit board 15. And they are passed, with this state maintained, through the reflow furnace 10, such as an infrared furnace or hot air furnace, to cause the cream solder to be melted. After soldering has been so achieved, the resultant components 13 are fixed in a predetermined position.
Since a high temperature is prevalent in the reflow furnace, an insulating material of the interconnection board 1 is softened or hardened to create a warp at the interconnection board 1. A deformation caused by such a warp in the reflow furnace 10 remains at the interconnection board even if the board is conveyed out of the furnace.
In a state of the interconnection board, etc., with electronic components not yet mounted thereon, a warp occurs at a thus manufactured circuit board and normally a time at which the manufacture of the board has been completed, except in the case of a flexible interconnection board, and there is more warp at the circuit board after soldering has been done at the reflow furnace where electronic components are mounted.
There is a risk that the warp at the interconnection board will cause a solder connection failure between the semiconductor leads mounted and the interconnection layers. The solder connection failure, if occurring, requires a manual connection operation and more manual steps of operation. Depending upon the cases involved, there may occur a stop in a line of production due to a drop of any warped interconnection board from a mounting device.
In accordance with the need for a more lightweight product, the circuit board becomes thinner and there is a strong tendency that more warp occurs in the mounting process of the electronic components on a thinner printed circuit board. And a higher percentage of faults occurs at the printed circuit board due to the warp at the thinner board. For this reason, an excessive amount of warp occurs, thus failing to allow the use of an automatic assembling apparatus upon the mounting of the electronic components at a subsequent step. This involves the cases where the components cannot be accurately mounted on the board.
There are sometimes the cases where a warp preventing jig may also be used as shown in FIG. 8 so as to prevent a warp in the reflow process. This jig is such as to provide a warp preventing fixture (3a, 3b) on both sides of a printed interconnection board 1 in a conveying direction in the reflow furnace or to provide a warp preventing bar, not shown, on the printed interconnection board 1.
If the jig is used, it is difficult to provide an automatic mounting of the jig. In this case, an operator""s manual operation is required at a mounting time. Even from the standpoint of costs, it is not desirable that the use of the warp preventing fixture involve an added manufacturing cost. More steps are undesirably required even in the case where the warp preventing bar is mounted as set out above.
An object of the present invention is to provide an interconnection board so designed as to have a balanced conductive pattern between respective layers in a conductive board and, by doing so, obtain a circuit board less likely to be warped even under an influence from heat in a reflow furnace, etc., and to provide an electric apparatus using a board less likely to be warped under a heat deformation.
In order to achieve the object of the present invention, conductive layers have at least a portion of a conductive member arranged in a nonlinear or polygonal configuration and having a greater layout area and an insulating layer is alternately stacked relative to the conductive layer, wherein a variation in amount of the conductive member at the conductive layer with a middle of a board thickness direction as a reference is set in a range in which a warp is less likely to be produced and in a range near to zero.
According to the present invention, in order to reduce a variation in amount of copper in a thickness direction of conductive layers in an interconnection board, the copper occupation rate of the conductive layer and thickness of the insulating layer are adjusted. By reducing the bending moment resulting from a heat deformation it is possible to prevent a warp of the interconnection board and to achieve less warp. It is possible to apply the present invention to a compact electric apparatus hard to achieve heat control and to contribute much to achieving a compact unit since it is not necessary to use other heat control component parts.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.